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Application of Direct Digital Temperature Control Systems forMaximum System Ef®ciency on VAV SystemsFrank W.Mayhew,EC/TC ConsultingThe acceptance of direct digital temperature control technology by the HVAC industry has exceeded theprojections of most manufacturers in the industry.Unfortunately,the application engineering of thesesystems has not been applied to their greatest advantage.Many of the control sequences are still appliedas if pneumatic controls were being used.Open-loop and indirect control loops are still employed for manyof the standard control functions that can now have closed-loop control.Integration between the major sub-systems has not been implemented to its full potential.This paper describes temperature control sequencesthat can be implemented with direct digital control that were not practical with discrete component pneumaticand electronic control systems.The purpose of these sequences is to operate the HVAC system with:(1) The greatest comfort possible,within basic design.(2) At the lowest energy cost while taking advantage of diversity of loads.(3) Closing open control loops and indirect control loops(4) Eliminating most of the energy waste of over-design,and correcting for some under-design of systemsby taking advantage of diversity.and pumps are controlled by their own individual controlsINTRODUCTIONand started and stopped with a time clock and outdoor airthermostat.Variable Air Volume (VAV) HVAC systems,by design,include taking advantage of diversity of loads in variousTypical morning warm-up for VAVsystems is a crude,bruteparts of the building,while lowering the basic cost of trans-force control sequence that forces open all the VAV boxesportation of the conditioned air and reducing peak loads.in the hope that enough return air will be circulated fromHowever,poorly applied temperature controls can negatethe warming exterior zones to the cooling-only interior zonesmuch of the potential saving of VAVsystems in commercialto adequately warm them for occupancy.buildings.Temperature controls,for VAV systems,havetraditionally been applied in sub-systems using discrete com-Because of the basic limitations on discrete component con-ponent pneumatic or electric control components.Mentaltrols,open-loop control (without direct feedback from theinertia is one of the reasons that advanced control systemscontrolled variable) is often provided using outside air tem-are not used and advanced control strategies are not imple-perature to reset supply air temperatures,mixed air tempera-mented.A Direct digital control (DDC) provides the oppor-tures and hot water and chilled-water temperatures.At best,tunity for the integration of sub-systems that work in directopen-loop controls are a compromise for both energy andresponse to load changes to eliminate many of the inef®-comfort,though they are better than nothing.Hot-water andciencies in HVAC system operation.chilled-water lockouts based on outside air temperaturerequire one to start systems too early and keep themrunningThe traditional implementation of pneumatic or electronictoo long.controls would be to install controls in three basic sub-systemsÐzones,air handlers and the central plantÐwith Indirect control loops (control loops that control a proxy forthe actual load) are found in almost every building wherelittle or no,integration between them.Feedback signals fromthe zones to the air handlers may be attempted,but is usually variable-¯owwater and air systems are employed,contribut-ing to inef®ciencies in building operation.Supply fan staticabandoned or negated when someone sets one of the thermo-stats up or down too far.Thermostats are installed in``repre- pressure is controlled by a pressure sensor located``2¤3ofthe way down the duct''and usually set much higher thansentative zones''to control morning warm-up,cool-downand night set-back strategies,but otherwise there is no further the VAV boxes require for proper operation.Differentialpressure sensors for secondary chilled-water pump controlattempt at integration of the sub-systems.Chillers,boilers,ApplicationofDirectDigitalTemperatureControlSystems-10.103are placed between the supply and return,near the end of pumping.The hot water systemis variable ¯owand variablethe piping loop,or at the secondary pump manifold,requiringtemperature.Fans and secondary chilled-water pumps areset-points that provide little or no turndown by the pump.provided with variable-speed drives to control speed.BackgroundDirect digital control provides the ®rst real opportunity tointegrate all of the sub-systems and to modify controlThe mechanical system designer is faced with the battlesequences to take advantage of diversity in part-load perfor-between designing a comfortable,energy-ef®cient buildingmance.The following are descriptions of how the variousand the capital costs of providing a truly energy-ef®cientcontrol loops would be controlled if discrete componentdesign that still can produce comfort within the bounds ofcontrols were used and how the control loops would bethe selected mechanical systems.Rarely is there a good life-modi®ed with direct digital control.cycle cost analysis done before signi®cant reductions aremade in the budget.The ripple effect is passed on to:theZONE CONTROLtenants who pay higher operating costs;the utility whichcannot build additional generation capacity except at greatThe zone control is really the most important element incost;and,eventually to the environment that must absorbany temperature control system.The zone,when within thethe additional 1.3 to 2.5 pounds per year of CO2per kWhrange of the zone controller,provides comfort.The air-consumed,if fossil-based fuels are used (Fischer,et al.handlers and central plant only provide the source and con-1992).version of the energy for HVAC,the zone is the user.If theair-handler and chiller are not properly controlled,the zoneScopewill either not be able to provide comfort,or will wasteenergy by throttling back.With this as the basic premiseA``typical''of®ce occupancy is examined to demonstratelet's look at the various control functions required for thehow an integrated control system can save energy whilebuilding and how DDC controls can improve performance.furnishing design comfort.The typical of®ce used will beZone controls can provide an immense amount of informa-a multistory building with multiple tenants.As with thetion that can be used as feed-back for integrated controlmajority of large of®ce buildings,the building is air condi-strategies.(Figure 2.)tioned using variable-air-volume,cooling only for the inte-rior zones,with re-heat for the exterior zones,a central airhandler,a centrifugal chiller and a central boiler (FigureMorning Warm-up1).The chilled-water system employs primary/secondaryUnder this category falls optimumstart control and morningFigure 1.Sub-Systems of a Typical Building Air Condition-warm-up and/or morning cool-down control.Cool-down ising System.similar to warm-up,but with cooling strategies instead ofheating.Some areas of the country need only warm-up,while warmer climates may need both.The intent is to haveFigure 2.Basic Control Block Diagram for Zone Feedbackto Fans and Central Plant.10.104-Mayhewthe building at a comfortable temperature in the morning at are not placed in the warm-up mode until the supply air iswarm enough to heat them.(3) Interior zones,instead ofoccupancy.Since the sample building is of multiple tenancy,the fans are started early enough for the ®rst tenant and being forced open,are instructed by the system to changecontrol action from direct acting to reverse acting,or vicethe entire building is operated,whether occupied or not.Amicroprocessor-based calendar-clock is typically used for versa so that these zones act like a heating terminal.Theminimum CFM is set to zero,and will shut down the airthis purpose.In a pneumatically controlled building thewarm-up routine is controlled by a thermostat located in a ¯ow when the zone is warm enough,and,(4) the set pointsfor the zone thermostats are all shifted down to 68×F to``representative''exterior zone that,when the temperatureis below its setpoint,starts the fan system with the outside prevent overheating of the zones.This strategy reduces thefan load and allows more fan capacity to warmup the coldestair dampers closed,the chiller off,the boiler and hot waterpump on and all the interior VAV zone boxes forced wide zones,while not using fan energy to try to warm up zonesthat are already warm enough for occupancy.The length ofopen to allow the return air to warm up the interior zones.The building is considered to be warmed up when the return time for morning warm-up can be reduced signi®cantly.air temperature is more than 68×F to 70×F.The``representa-tive''zone is often chosen during construction and may notMorning warm up works better if there is a small heatingbe``representative''at all,complicating the process.coil in the main supply duct to warm up the air for theinterior zones.Attempting to warm up the interior zonesThis method of morning warm-up is inadequate in severalwith fan energy added the return air from the exterior zonesways.The intent of such a system is to move the warm airwastes energy since the fan energy by itself does not signi®-fromthe exterior zones that have re-heat to the interior zonescantly raise the supply air temperature and warm-up willvia the re-circulation of return air.It takes time for thetake longer.exterior zones to warm up the return air enough to provideany signi®cant heating to the interior zones and,if the interiorOnce the building has been warmed up,those interior zoneszones are already at a satisfactory temperature,this methodthat have a later occupancy can be turned off to save fanmay even sub-cool the interior zones resulting in poor com-energy,or placed in a standby mode.Warm-up for exteriorfort at occupancy time.zones of the building can be delayed until the system calcu-lates the need for morning warm-up based on their actualDuring morning warm-up,the supply fan cannot adequatelyoccupancy,not the occupancy of the earliest tenant.serve all the zones if the boxes are all wide open,becausea fan for a VAV system is designed for diversity of zonesBoiler Start/Stopand is not designed to serve all zones with full ¯ow simulta-neously.With reduced air¯ow,diffuser velocities areIn the conventional control system the boiler is stoppedreduced,causing the air to short-circuit to the ceiling returnbased on time-of-day and/or if the outside air temperaturegrilles,not providing any signi®cant heating to the space.is over some ®xed temperature,usually 70×F,or higher.The fan may not even be able to get air to the zones farthestOnce a building is warmed up,and the outside temperaturefrom the fan because the zones closest to the fan use all theis above the thermal balance point of the building,the boilerair,and starve the rest of the system.can be shut off based on all the zone re-heat valves beingclosed.Typical building thermal balance points are betweenAreturn air thermostat may not be representative of whether50×F and 60×F.With a large number of hours between 50×Fthe building has warmed up suf®ciently due to the short-and 70×F,running the heating system longer amounts to acircuit of air¯ow in the exterior zones and the averaging ofconsiderable amount of energy.The feedback fromthe zonethe air from many zones together.Energy is wasted byre-heat valves will also allowautomatic starting of the boileroperating the fan at excess volume and,because the morningand pump if the need for heat should arise.Note that if thewarm-up is ¯awed,the operators will invariably extend theminimum CFM's on exterior zone boxes are set too high,time of the morning warm-up,starting earlier and earlierthe boiler and hot water pump run-time will be extended upuntil the complaints stop,thus wasting additional energyto 85×F in some cases observed by the author.without doing any real work.Heating Hot Water ControlA direct digital control system offers the opportunity towarm up the building using temperature sensors in all thezones of the building.A calculation program is still used to Most discrete component control systems employ some formof hot water reset based on outside air temperature in orderdetermine the optimum time to start warm-up,but there aresigni®cant differences from the discrete component control to approximate the building load and to improve the controlresponse of the re-heat hot water valves and reduce heatmethod.(1) If a zone does not require morning warm-up,that zone stays out of the warm-up mode.(2) Interior zones loss through the insulation of the circulating system.WithApplicationofDirectDigitalTemperatureControlSystems-10.105an integrated control system,the hot water reset can be than install and control isolation dampers or other automaticmeans to serve only the occupied area.With the control oflocked out during morning warm-up to speed up the warm-up process.During normal occupancy hours the hot water all of the zone boxes,occupancy control can be achievedeasily by manual entry into the central terminal,by push-reset can be reinstated using the re-heat valve positioninstead of outside air temperature as the reset input.This button overrides that give a predetermined number of hours,or minutes of override,or by a telephone interface systemwill provide the lowest temperature hot water needed forthe re-heat valve calling for the most heating to satisfy the that allows occupants to access overrides by a telephoneoverride system.Telephone based systems allow the mostspace conditions.¯exibility,as they allow the tenant to schedule overrides inBecause two-way valves are used on the heating coils andadvance,and the building owner can receive printouts ofthe hot water temperature is varied during occupied hours,itoverride activity to use in a revenue-generating programis probably not cost effective to use variable-speed pumpingwhere billing for after-hours HVAC use is included in thebecause as the temperature of the hot water is lowered,thelease.volume goes up to provide the same quantity of heatingenergy.With feedback from the zones,the operating timeSUPPLY FAN STATIC PRESSUREon the hot water pump will be minimal except in colderCONTROLclimates where a variable speed pumping system may beof value.Static pressure control of supply fans for variable-air-volumesystems is probably one of the more misunderstood controlOccupancy Controlloops in an HVAC system.The common practice of placingthe static pressure sensor``2¤3of the way down the duct``One of the greatest opportunities for energy saving is missedis not an energy-ef®cient method of control since the controlwith discrete component control systems by not institutingset points tend to be set at 1.0 to 2.0 inches w.c.(Figuresome form of occupancy control.The lighting control side3.).The higher the static pressure control point,the less totalof many buildings now includes occupancy sensors to turnfan static is available for turndown.In high rise buildingsoff the lighting in unoccupied spaces as an energy conservingwith central fan systems,the problem is exacerbated by themeasure.The potential savings in a VAVsystem,with occu-practice of placing the static pressure sensor in the mainpancy control,include fan,chiller and boiler energy.Withduct serving the ¯oors.When you have a fan design withintelligent control at the zone level,there are several opportu-4.0 in.w.c.and a control point of 2.0 in.w.c.,the fan doesnities to implement occupancy-based control systems.not have the full opportunity to take advantage of fan af®nityThe ®rst case is the casually unoccupied zone.A simpleinterface between the lighting occupancy sensors (if used)Figure 3.Static Pressure Control Location.and the zone controls can make a signi®cant contributionto energy conservation.If you take the average breaks thatan of®ce worker uses,lunch,coffee breaks and trips to therest rooms there is at least 1.5 hours,out of a nine houroccupancy period,that the of®ce is not occupied.Just thelunch hour alone is 11% of the nine hour day.Add to thismeetings and absences for sick days and it is easy to reach20%vacancy factor in most occupancies during the workday.Auxiliary contacts are available on some low-voltage light-ing relays that can be connected to the VAV box controllerto force the box closed when unoccupied.Where severallighting zones are combined with one HVAC zone,the con-tacts can be wired in series.The second case is when a building is occupied by multipletenants with varying time schedules.This can be a singlecompany with various departments,or multiple companies.It is rare that the design includes control sequences with theability to handle after-hour occupancy.When a buildingoperator is faced with one tenant or department that workslater than the rest,the systems are generally operated tocondition the whole area that is served by the system rather10.106-Mayhewlaws.The lower the control pressure,the closer the supply for the static pressure sensor?At the end of the duct runallows for the lowest set point for the static pressure control,fan energy will follow the fan cube law.The return fan,forinstance,already closely approaches the fan cube law but the most ef®cient method of static pressure control isusing the VAV box damper position to feedback to thebecause the relative control point is only 0.05 in.w.c.,thebuilding pressurization.The 1995 ASHRAE Applications variable speed drive control.Handbook states that the static pressure sensor should belocated``at 75% to 100% of the distance between the ®rstBut what do pressure independent VAVboxes really requireand last terminal''and that``the pressure selected provides(Figure 4.)?And when?The manufacturers of VAV boxesthe minimum static pressure to all air terminal units duringusually rate the boxes at maximumCFMincluding an allow-all supply fan design conditions.''ance for 0.1 in.w.c.of downstream resistance.Note thatthis requirement is at maximum rated CFM.Most VAVboxes are selected at some rating less than maximum ratedThe proof is in the formulas that determines the static pres-CFM,about 80%of maximumrating to reduce noise genera-sure at the fan and the brake fan horsepower:tion and to allow some margin for error.At 80%CFM,withthe rated static pressure the box would be approximatelySFan4CFM2x (SDes1SConti)`SContr80%open,introducing a resistance into the systemto throttlethe ¯ow.If the upstream static pressure was reduced toWhere0.32 in.w.c.(64% of required minimum),the pressure-independent VAV box would sense the reduced ¯ow andSFan4Static of the fanwould open further to maintain the ¯ow the thermostat iscalling for.As the load decreased further to 50%,the boxSDes4Design static of fanwould begin to close off but if,at the same time,if the ductstatic pressure is lowered,the box could be fully open at 50%SContr4Static Pressure Control Pointload,with less than 0.1 in.w.c.static pressure at the inlet.BHPfan4~CFM x Static PressureFanEffFanx 6359!The solution is to abandon the duct static pressure controlof supply fans and to replace it with direct feedback fromthe VAV box damper positions.With most manufacturersNote that the static pressure control point directly affectsof DDC VAV controllers,one of the items of informationthe brake horsepower of the fan.available is the box damper actuator position.With DDCsystems,it is relatively easy to collect the box damper posi-Zone Control of Static Pressuretions and select the box with the damper that is open themost.The setpoint for the variable-speed drive becomesThe variability and diversity of the CFMdemand of a vari-VAV box damper position,i.e.,95% of damper opening.Itable-air-volume system is determined by solar exposure,is important in developing the control loop to feed the boxoutside air temperature and interior load.All these factorsposition signal into an averaging function to slow down thein the load vary throughout the operating day.As the suncontrol action.Averaging the readings over ®ve minutesmoves around the building,the primary load moves fromshould be suf®cient,in most cases.the east to the south and then to the west.Cool morningsbecome warm afternoons and people come and go in theinterior spaces,turning lights and equipment on and off,andFigure 4.The Pressure Required of the Supply Fan is theopening and closing window coverings.The result is that itPressure Required to Push Design CFM Through any Boxis impractical to ®nd any single,or even multiple locationsRequiring Design CFM.for static pressure sensing in the ducts that will be right forall loads.In addition the single setpoint control loop is reallyan indirect control loop,just like outdoor reset of hot watertemperature.The actual load is the CFMthat the VAV boxmust provide to the zone.There must be enough pressurein the duct to see that every VAV box in the system canprovide the required CFMto the zone for the load at that time.Integrated direct digital control provides a better methodof controlling fan capacity (Englander and Norford,1992,Hartman,ASHRAE,1992).The problemis best shown withthe duct arrangement in Figure 3.Where is the best placeApplicationofDirectDigitalTemperatureControlSystems-10.107Using a direct-acting control loop with the box calling for pumping system is wasted by either using a bypass valveto control differential pressure or by using differential con-the most cooling at 95% open,if the load increases in thatzone,the box will open more.As the box opens beyond trol of the variable-speed drive but selecting the wrong loca-tion for the control sensor.The differential pressure control95%,the supply fan VFD will be commanded to speed upuntil the box is back to 95%.As the load is reduced or boxes sensors are often found a few feet away from the secondarychilled-water pumps set at 30 psid.The more ef®cient instal-are closed off at the end of occupancy,the fan slows downuntil at least one box is 95% open.lations put the differential pressure sensor between the sup-ply and return,toward the end of the chilled-water loop setat about 10 psid.Just as with the supply fan,the controlPotential savings are on the order of 20% to 40%of the fansetpoint is critical to the amount of saving available from aenergy if the VAV boxes toward the end of the duct are thevariable-¯ow pumping system.The setpoint is selected toones that are usually calling for the most cooling.In theensure that there is enough differential pressure at full ¯owcase of the loop duct,as the sun travels around the buildingto provide design ¯ow through the control valve,the coil,the load will follow and the boxes closer to the fan willand associated piping.require the most pressure,reducing the overall fan energyeven more.In addition to energy saving,the radiated andAn analysis of the system will show that the control valvedistributed noise generated from VAV boxes when the vol-is very similar to the VAV box and is selected based onume damper modulates toward the closed position,will bepressure drop at full ¯ow.Two-way chilled-water valvesreduced through most of the operating range.are typically selected with 1 to 2 psi drop at full ¯ow,thoughoccasionally they are selected at up to 5 psi.The chilled-Chiller Start/Stopwater coils are also selected with approximately 5 psi dropat full ¯ow.While it is readily apparent that the coil pressureStarting and stopping the chiller has usually been a time anddrop varies with the square of the ¯ow,the control valvetemperature based control with discrete component controlalso has the same characteristic when it is open fully.systems.By analyzing the system we can ®nd an alternatestrategy that is even more ef®cient and will prevent theTo take full advantage of the turndown capability of a vari-chiller and its associated equipment from operating on mildable ¯ow secondary pumping system the control systemdays.When is the chiller needed?When a zone(s) runs outmust try to maintain at least one chilled-water control valveof cooling capacity.When is a zone out of cooling capacity?95% open.The control logic is the same as for the supplyWhen the zone damper is fully open.But,a zone damperfan static pressure control,using the information availablecan be open for two reasons,the supply duct pressure canin the direct digital control system to select the valve thatbe too low,or the air temperature in the supply duct is notis open the most and to reduce the pressure until that valvecold enough.Since our systemis already using zone damperis 95%open.Because the supply air temperature loop is notposition to control the supply fan capacity,then we knowcontrolling ¯ow like the pressure-independent VAV box,that if a damper is fully open,and the variable speed drivethe time lag for control action may have to be increased tocontrolling the supply fan is at,or near,full speed,then theapproximately 10 minutes to allow time for the supply airtemperature of the air is too warmfor cooling and the chillertemperature loop to detect the change in ¯ow through theneeds to be turned on.This gives the information necessarycoil.It should be noted that this control sequence will onlyto provide an optimum start time for the chiller only whenwork if the chilled-water control valve is controlled by athere is need for chilled water.supply air sensor.The time lag in room sensors controllingchilled-water valves is too long be responsive to this typeThis strategy trades fan energy for chiller energy by notof control system.starting the chiller and associated pumps at 55×F outside airtemperature,by keeping the chiller off until the outside airThis control method will solve some of the problems oftemperature is near 60×F,and should save 500 to 1,000 hourspressure distribution and reduce the need for balancing inper year of chiller operation over a chiller that is startedvariable ¯ow water systems (Avery,1992).only on the basis of outside air temperature and time-of-day.Other StrategiesSecondary Chilled-Water Pump ControlWith global control of the zones,other strategies can beemployed to reduce energy use.Zone temperature set-pointsUsing a primary/secondary chilled-water loop is anothermeans to reduce the energy use of the building.In multiple- can be shifted up on warm days and down on cold days.Atthe end of the day,set-points can be shifted up to reducechiller operation,well-controlled secondary loops provide ameans for staging on and off chiller capacity.Too often chiller and/or fan energy.This brings up another strategy,avoid using proportional-integral control loops for zones.much of the energy savings from a secondary chilled-water10.108-MayhewUsing a proportional-only loop will allow the zone tempera- and ESCO's think that the mere installation of a full DDCsystem automaticaly gives them all the energy ef®ciencyture to droop when the load is light and rise when the loadis heavy.Since much of the load variation on the exterior strategies possible.This is not true and won't be true untilall the participants in the project learn the capabilities of thezones is weather related,the zone temperature will automati-cally shift toward the outside air temperature lowering the control systems and learn to ask for all the features thatthey want or need.A good study of all the HVAC systemlosses,or gains through the outside surfaces.components is required to be able to determine what controlstrategies will work and will save energy,while providingIf the telephone interface is used for after-hour occupancycomfort.control,occupants can use the system to have control overthe zone set points,but the range of setpoint adjustment canControl and mechanical designers need to get away frombe limited to reduce the impact on feedback systems.``the way we've always done it''and truly analyze the waythe systems operate and use energy.Computer based controlsOTHER ISSUESprovide the means to bring about true energy ef®cient controlof HVAC.However careful programming and completeFurther work is needed to implement technologies likecommissioning are still necessary for an energy ef®cient and``fuzzy logic''to the control systems that can make eco-comfortable air conditioned building.One building,beingnomic decisions between various control strategies.Forused in an ASHRAE research project,saved over 40% ofinstance,is it more ef®cient to start the chillers or to usethe energy they were using for HVACjust by tuning a``statefan energy for cooling.There are times when resettingof the art''system.This points up the need for understandingchilled-water temperatures up may be more energy ef®cientthe systems that controls are being applied to and not takingthan the increased pumping energy to handle the higherfor granted that once a systemis installed that it will operate¯ows.as planned.CONCLUSIONREFERENCESThe installation of direct digital control to all controlledFischer,Fairchild and Hughes,1992.Global Warming Impli-components in HVAC systems provides the opportunity tocations Of Replacing CFC's.ASHRAE Journal,April.achieve a higher degree of energy ef®ciency than with dis-crete control components.The resulting integration of sub-Englander and Norford.1992.Saving Fan Energy In VAVsystems reduces operating times and increases ef®ciencySystems-Parts 1 and 2.ASHRAE Transactions 98(1).of operating equipment.For the highest degree of energyef®ciency all systems must be integrated and operate inG.Avery.1992.Should Variable-Flow Pumping Systemsconcert,the zones being the most important because theyBe Balanced.ASHRAE Winter Meeting,1992,Anaheim.are the load being served.Do not assume,however,that justASHRAE Transactions 98(1).because you have installed a DDC system that the controlstrategies that are implemented include the ones discussed T.Hartmann,Terminal Regulated Air Volume Control.ASHRAE Winter Meeting,Chicago,1991.in this paper.Many building owners,mechanical designers,ApplicationofDirectDigitalTemperatureControlSystems-10.109